Mass spectrometry (MS) is a widely used technique in academic research as well as in industry for the analysis of various compounds. Up to now, a broad range of detection methods and ionization processes were developed and therefore only those applied in this work will be discussed in a simplified fashion.98,99
The basic principle of a mass spectrometer can be divided into different parts (Figure 2-2).98 First of all the sample has to be introduced into the mass spectrometer. In mass spectrometry the conversion of the analyte into the gas phase as well as its ionisation is required. The resulting gaseous and ionized compounds are separated based on their mass-to-charge (m/z) ratio and subsequently analysed by a detector. The ionisation process, the separation and the detection of the sample are carried out in vacuo.
Figure 2-2. Simplified set-up of a mass spectrometer.
Basics of ESI-MS & MALDI-TOF-MS
Within the field of ionisation procedures electrospray ionisation mass spectrometry (ESI-MS) and matrix-assisted laser desorption mass spectrometry (MALDI-MS) are two of the most established techniques.
These soft-ionisation methods were awarded with the Noble Prize in the year 2003.100,101
In case of ESI, the sample has to be dissolved homogenously using a polar and volatile solvent (for further discussion see chapter 5). This solution is transferred through a capillary with a decreasing diameter. The capillary is charged with a high voltage via an electrode tip and the analyte is sprayed into the mass analyser. This procedure results in highly charged droplets.102 Through repulsion electric forces as well as the evaporation of the solvent a downsizing of the droplet occurs until a unsolvated single analyte molecule is obtained which is analysed (Figure 2-3).98,99,103 In contrast to ESI, in MALDI-MS the analyte is supported by a matrix in the solid state. Therefore, the choice of a suitable matrix and a careful sample preparation are fundamental in MALDI.19 Often organic molecules with an aromatic -system are applied in combination with an acid that is able to ionize the analyte during the MALDI process.99,104 The matrix is irradiated with using a laser.99 The absorption of the laser energy leads to local heating and the matrix as well as the ionized analyte are transferred into the gas-phase.104–106 Via an applied accelerating voltage the analyte is introduced to the detector system (Figure 2-4).106
Figure 2-3. Simplified mechanism of electrospray ionisation.98
Figure 2-4. Simplified mechanism of matrix-assisted laser desorption ionization.
The mass analyser used in this work is based on the separation of the ions through an applied electric field. Via this field the ionized analytes were accelerated and due to the diversity in the m/z-ratio a difference in the (electric-field-free) flight duration occurred. This leads to a separation of the analytes followed by their detection. This technique is called time-of-flight mass spectrometry (TOF-MS).107
Mass Spectrometry of Polymers and Polyethylene
Mass spectrometry has a wide range of applications.98 In the field of polymer characterization mass spectrometry is well-established.20 Due to the high sensitivity of MS, distinct chains are separated and well resolved spectra and mass distributions can be achieved.20,108 Consequently, both MALDI-MS and ESI-MS offer the possibility for a detailed end-group characterization of polymers.19 It has to be considered that the obtained spectra of ESI and MALDI can differ, e.g. in case of ESI multiple charged chains whereas in MALDI only single charged molecules can be detected.
In addition to end-group characterization, mass spectrometry gives access to molecular weight distribution, although a divergence compared to molecular weight distribution obtained from site-exclusion chromatography (SEC) is observed.19,108 This is explainable because of mass discrimination effects in MS. Polymers with higher molar masses are more difficult to evaporate and the transport efficiency of such polymers is reduced. For this reason, molar mass distributions obtained from MS are typically shifted to lower molar masses compared to SEC.109–
111 The most established route for the formation of polymer ions is the adductions of auxiliary ions like alkali metals (specially Na+) in case of hydrophilic polymers or silver ions for more nonpolar polymers like polystyrene bound via -interactions.108,109
Due to the lack of polar groups in polyolefins, especially for PE, mass spectrometry is still a challenging topic and is commonly not suitable for these classes of polymers.21 Nevertheless, one possibility to overcome this limitation is the introduction of polar or vinylic groups into PE. Polar groups can be introduced e.g. by using catalyzed chain growth (chapter 2.3). These polar groups offer a coordination site and can be used in MALDI-MS via the explained metal adduct method.93,97 Vinylic terminated polyethylene can be synthesized using conventional polymerization techniques via -hydride elimination (see chapter 2.2, Scheme 2-5). These double bonds can be ionized by using e.g. silver ions and therefore partially be detected in MALDI-MS.112 Another approach, that builds the basis of this work, is the incorporation of a charged group into polyethylene, mainly developed by Wallace and coworkers.113,114 The synthesis of precharged polyethylene gives direct access to MALDI-MS without any additional metal adducts.21,114 The synthetic procedure is
subsequently brominated followed by a nucleophilic substitution using triphenylphosphine yielding a permanent charged PE (Scheme 2-9). The overall yields obtained in this procedure were reported to be around 48 %.21,113 In addition to the work of Wallace, by the introduction of other charged groups or using a cationized catalyst-bound intermediate, a few examples are described in literature for the analysis of polyethylene via ESI-MS that is typically more difficult due to the requirement of a homogenous solution as described above.115,116 A detailed discussion of charged polyethylene, also its usage for other applications, will take place in chapter 5.
Scheme 2-9. Simplified reaction scheme for the synthesis of charged functional PE based on the work of Wallace and coworkers.